Electrophysiology of Ctenophore Smooth Muscle

Unlike in the Cnidaria, where muscle cells are coupled together into an epithelium, ctenophore muscles are single, elongated, intramesogleal structures resembling vertebrate smooth muscle. Under voltage-clamp, these fibers can be separated into different classes with different sets of membrane ion channels. The ion channel makeup is related to the muscle's anatomical position and specific function. For example, Beroe ovata radial fibers, which are responsible for maintaining the rigidity of the body wall, generate sequences of brief action potentials whereas longitudinal fibers, which are concerned with mouth opening and body flexions, often produce single longer duration action potentials.Beroe muscle contractions depend on the influx of Ca²⁺. During an action potential the inward current is carried by Ca²⁺, and the increase in intracellular Ca²⁺ concentration generated can be monitored in FLUO-3-loaded cells. Confocal microscopy in line scan mode shows that the Ca²⁺ spreads from the outer membrane into the core of the fiber and is cleared from there relatively slowly. The rise in intracellular Ca²⁺ is linked to an increase in a Ca²⁺-activated K⁺ conductance (K_Ca), which can also be elicited by iontophoretic Ca²⁺ injection. Near the cell membrane, Ca²⁺ clearance monitored using FLUO3, matches the decline in the K_Ca conductance. For light loads, Ca²⁺ is cleared rapidly, but this fast system is insufficient when Ca²⁺ influx is maintained. Action potential frequency may be regulated by the slowly developing K_Ca conductance.